Method and apparatus for an in-rush current limiting circuit

ABSTRACT

An in-rush current limiting circuit is disclosed. An apparatus according to aspects of the present invention includes a power switch having a first, second and third terminals. A capacitor having a first terminal and a second terminal is also included. The second terminal of the capacitor is coupled to the first terminal of a current source. The second terminal of the current source is coupled to a second input terminal of the in-rush current limit circuit. A power switch is also included. The first terminal of the power switch is coupled to the anode of a diode. The cathode of the diode is connected to the first terminal of the current source. The second terminal of the power switch is coupled to a second input terminal of the in-rush current limit circuit. The third terminal of the power switch is coupled to be responsive to a voltage across the current source circuit in response to a rate of change of voltage between first and second terminals of the power switch.

REFERENCE TO RELATED APPLICATION

The present application is a continuation of U.S. patent applicationSer. No. 11/393,026, filed Mar. 30, 2006, now pending, entitled “METHODAND APPARATUS FOR AN IN-RUSH CURRENT LIMITING CIRCUIT,” which is herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to in-rush current limitingcircuits, and more specifically, the present invention relates to lowdissipation in-rush current limiting circuits.

2. Background Information

In certain applications of electronic circuits, it is desirable tocontrol the in-rush current that flows into a circuit when a supplyvoltage is applied. Examples of such applications are in DC-DCconversion (or rectified AC) applications where a power conversion unitis connected to a DC source (or rectified AC) supply and where the powerconversion unit includes a large capacitor at the input to the powerconverter. The in-rush current limiting circuit function is to regulatethe current drawn from a power source providing the DC supply voltage tothe power converter circuit. Without the use of an in-rush currentlimiting circuit, the current that charges the input capacitor of thepower converter would be uncontrolled leading to a very high in-rushcurrent potentially causing damage to connectors and other components ofthe power converter and power source equipment.

Applications for in-rush current limiting circuits are normally requiredto operate at high efficiency and therefore any in-rush current limitingcircuit configuration allowing improved efficiency is highly desirable.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention aredescribed with reference to the following figures, wherein likereference numerals refer to like parts throughout the various viewsunless otherwise specified.

FIG. 1 is a block diagram illustrating generally an example of anin-rush current limit circuit in accordance with the teachings of thepresent invention.

FIG. 2 is a schematic diagram illustrating generally an example of anin-rush current limit circuit in accordance with the teachings of thepresent invention.

FIG. 3 is a schematic diagram illustrating generally another example ofan in-rush current limit circuit benefiting from the teachings of thepresent invention.

DETAILED DESCRIPTION

Examples of apparatuses and methods for implementing an in-rush currentlimiting circuit are disclosed. In the following description, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. It will be apparent, however, toone having ordinary skill in the art that the specific detail need notbe employed to practice the present invention. Well-known methodsrelated to the implementation have not been described in detail in orderto avoid obscuring the present invention.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the present invention. Thus, the appearances of thephrases “in one embodiment” or “in an embodiment” in various placesthroughout this specification are not necessarily all referring to thesame embodiment. Furthermore, the particular features, structures orcharacteristics may be combined for example into any suitablecombinations and/or sub-combinations in one or more embodiments.

An improved in-rush current limiting circuit and method for implementingsuch a circuit in accordance with the teachings of the present inventionwill now be described. Examples of the present invention involve methodsand apparatuses to simplify and improve an in-rush current limitingcircuit such that no current sensing element is required and thelimiting of in-rush current is performed by controlling the rate ofchange of a voltage across the drain and source terminals of a powerswitch, which may also be referred to as controlling the slew-rate ofthe voltage across a power switch in accordance with the teachings ofthe present invention.

To illustrate, FIG. 1 shows generally a block diagram of one example ofan in-rush current limiting circuit 101 in accordance with the teachingsof the present invention. As shown, in-rush current limiting circuit 101includes a power switch 104 coupled to a capacitor Ct, which is coupledto a current source 102 and a diode Ds, coupled to the power switch 104.The current source 102 is coupled to an input terminal of the in-rushcurrent limiting circuit 101 and the power switch 104 is also coupled tothe same input terminal of the in-rush current limiting circuit 101. Inthe illustrated example, the gate terminal of power switch 104 iscoupled to power switch control circuit 103, which is coupled to receivean input supply voltage 105 from one of the input terminals and a signalfrom a sense node 106 between current source 102 and capacitor Ct. Theanode of diode Ds, is coupled to the Drain of the power switch 104 andthe cathode of diode Ds is coupled to the capacitor Ct and currentsource 102.

As will be discussed, the gate terminal of power switch 104 is coupledto be responsive to the voltage across the current source 102 and inresponse to a rate of change of voltage across the power switch 104 viapower switch control circuit 103 in accordance with the teachings of thepresent invention. In one example, this is achieved with the signalapplied to the gate terminal of the power switch 104 from power switchcontrol circuit 103 being responsive to the voltage across the currentsource and in response to a change in a voltage across the power switch104 in accordance with the teachings of the present invention.

To illustrate, the in-rush current limiting circuit 101 is coupled toreceive a direct current (DC) supply voltage from a power source coupledbetween the input terminals of the in-rush current limiting circuit 101.The circuit of FIG. 1 employs the power switch control circuit 103 thatreceives the input supply voltage signal 105, which may be used todetermine an input supply voltage threshold voltage at which the turn onof a power switch 104 and turn on of current source 102 are initiated.The power switch 104 and current source 102 are therefore off until athreshold value of the input supply voltage signal 105 is reached.

When the supply voltage is first applied to the in-rush current limitingcircuit 101 of FIG. 1, and until threshold value of the input supplyvoltage signal 105 is reached, the supply voltage is substantially alldropped across the drain-source terminals of the power switch 104. Whenthe current source 102 turns on, it allows diode Ds to conduct. Theanode of the diode Ds is at the voltage of the power-switch Drainterminal. The cathode of diode Ds is one diode drop below thepower-switch Drain terminal voltage, when the current source 102 isoperating. When the power switch control circuit 103 starts to turn onthe power switch 104, the drain-source voltage across the power switch104 begins to fall. The cathode of diode Ds maintains one diode dropbelow the value of the power switch 104 Drain terminal. This change involtage across power switch 104 from the drain terminal to the sourceterminal of power switch 104 is thereby coupled via diode Ds to createsa current flow in capacitor Ct.

The change in voltage on the capacitor Ct is coupled to the power switchcontrol circuit 103 via the sense node 106 between current source 102and capacitor Ct to the power switch control circuit 103. The powerswitch control circuit 103 then controls a gate drive to the powerswitch 104 to regulate the rate of voltage change in capacitor Ct thuslimiting the voltage slew rate across the power switch 104 and thereforealso the in-rush current flowing in output bulk capacitor Cb inaccordance with the teachings of the present invention.

It is appreciated that a resistor can be used to replace current source102. The use of a controlled current Ic provided by current source 102,however, has advantages over using a simple resistor in place of currentsource 102, since the current source 102 current Ic is insensitive tothe value of the supply voltage and can be made insensitive totemperature effects that could otherwise influence the value of currentflowing in Ct at which the sense node 106 generates a signal that thepower switch control circuit 103 can detect in order to regulate theflow of current in capacitor Ct. The use of a current source 102 alsoallows accurate control of the slew-rate of the drain of the powerswitch 104. It is appreciated that in another example, the metal oxidesemiconductor field effect transistor (MOSFET) power switch 104 of FIG.1 could be replaced by a bipolar transistor in accordance with theteachings of the present invention.

FIG. 2 shows generally a schematic of an example in-rush currentlimiting circuit 201 in accordance with the teachings of the presentinvention. As can be observed, it is noted that example in-rush currentlimiting circuit 201 of FIG. 2 shares some similarities with the examplein-rush current limiting circuit 101 of FIG. 1. In the example shown inFIG. 2, the elements of the power switch control circuit 203 are shownin more detail. As shown, Zener diode VR1 is coupled to receive theinput supply voltage signal 205. In the illustrated example, the Zenerdiode VR1 Zener voltage V_(T) determines a threshold value of the inputsupply voltage from the power source at which the power switch 204 andcurrent source 202 start to turn on.

In the example shown in FIG. 2, the sense node 206 is coupled to thebase of transistor Q1. The emitter of transistor Q1 is coupled to acapacitor Ct. The cathode of diode Ds maintains one diode drop below thevalue of the power switch 204 Drain terminal. A change in voltage acrosspower switch 204 from the drain terminal to the source terminal of powerswitch 204 is thereby coupled via diode Ds. The base of transistor Q1 iscoupled to the current source 202 and diode cathode Ds. When the drainvoltage of the power switch 204 lowers, so also does the voltage on thebase of transistor Q1, thereby turning it on. Transistor Q1 conducts thecurrent charging Ct which also flows through resistor R2 thereby drivinga voltage. When the voltage across resistor R2 reaches the base emitterthreshold voltage of transistor Q2, transistor Q2 turns on, which pullsthe gate voltage of the power switch 204 down, which tends to turn thepower switch 204 off.

In the illustrated example, the following equations apply:

$\begin{matrix}{I_{IN} = {C_{b} \cdot \frac{\Delta\; V_{C_{B}}}{\Delta\; t}}} & \left( {{Equation}\mspace{14mu} 1} \right) \\{I_{IN} = {C_{b} \cdot \frac{\Delta\; V_{{POWER} - {{SWITCH}\mspace{14mu}{({DRAIN})}}}}{\Delta\; t}}} & \left( {{Equation}\mspace{14mu} 1} \right) \\{I_{IN} = {\frac{C_{b}}{C_{t}} \cdot \frac{\Delta\; V_{Q\; 2{({be})}}}{R_{2}}}} & \left( {{Equation}\mspace{14mu} 2} \right)\end{matrix}$Equation 1 calculates the in-rush current as a function ofbulk-capacitor Cb and the rate of change of bulk-capacitor voltage.Equation 2 shows that in-rush current can also be expressed as functionof the slew-rate or rate of change of power switch drain voltage.Equation 3 shows that in-rush current can also be expressed as functionof bulk capacitor Cb, capacitor Ct, and the base-emitter voltage drop oftransistor Q2 along with the value resistor R2.

The gate drive of the power switch 204 is therefore responsive to asignal at the sense node 206, the sense node 206 being the node couplingcapacitor Ct to current source 202. The gate drive of the power switch204 is therefore also responsive to the change in voltage across thecurrent source 202 since it is this signal that is applied to the baseof transistor Q1 and thereby also couples to drive resistor R2 andtransistor Q2. Zener diode VR2 limits the gate voltage of the powerswitch 204 after the in-rush function has been completed and the powerswitch 204 drain-source voltage has reached a steady state valuedetermined by the on resistance of the power switch 204 and the rate ofchange of the drain-source voltage of the power switch 204 issubstantially zero.

FIG. 3 shows generally schematic of another example in-rush currentlimiting circuit 301 in accordance with the teachings of the presentinvention. As can be observed, it is noted that example in-rush currentlimiting circuit 301 of FIG. 3 shares some similarities with the examplein-rush current limiting circuit 201 of FIG. 2 and/or in-rush currentlimiting circuit 101 of FIG. 1. In the example shown in FIG. 3, theZener diode VR5 conducts when the input supply voltage signal 305voltage exceeds approximately 27V. This generates a voltage acrossresistor R24 driving the gate of the power switch 304 MOSFET Q9 andactivating the current source 302. When the gate voltage reaches thepower switch 304 MOSFET Q9 turn-on threshold, power switch 304 andcurrent source 302 will start to turn-on.

In turning on, the power switch 304 drain voltage will start to fallbelow the supply voltage potential. This fall in power switch 304 drainis coupled to the cathode of diode Ds. The fall in potential turns ontransistor Q13 and Q13 conducts charge current from capacitor C1 to theresistor R27. It is noted that capacitor C1 is equivalent to capacitorCt in FIGS. 1 and 2. It is also noted that the current source formed byR32, R33 and Q11 is equivalent to current sources 102 or 202 of FIGS. 1and 2, and is only one example of a current source circuit which may beemployed in accordance with the teachings of the present invention. Inother examples, the current source formed by R32, R33 and Q11 could bereplaced with another suitable type of current source (e.g. including asimple resistor) in accordance with the teachings of the presentinvention.

In the example illustrated in FIG. 3, the current source R32, R33 andQ11 maintains voltage on the cathode of diode D1 at one diode drop belowthe voltage of the drain of the power switch Q9. As the power switch Q9turns on, the drain voltage of the power switch Q9 will lower andthereby lowering the cathode of diode D1 and the base of transistor Q13at sense node 306, turning transistor Q13 on. The sense node 306 signalis therefore responsive to the voltage across the current source formedby R32, R33 and Q11. The transistor Q13 conducts charge current fromcapacitor C1 and generates a voltage signal across the lower resistorR27. The voltage signal across R27 drives the base of transistor Q10causing it to conduct, thus pulling down on the gate of the power switch304 MOSFET Q9 tending to turn power switch 304 off.

In the illustrated example, a constant charge current flowing incapacitor C1 and through resistor R27 is proportional to a constant rateof change in the power switch 304 MOSFET Q9 drain voltage, which is thedrain voltage slew-rate of power switch 304. Hence, by controlling thecapacitor C1 charge current through resistor R27, the drain-voltageslew-rate of power switch 304 is also controlled. Also the diode Ds inconjunction with current source 302 allows the cathode of diode Ds tofollow the voltage potential of the power switch 304 drain. Thus aclosed loop system is completed such that the control terminal signal,or gate drive signal, applied to power switch 304 MOSFET Q9 isresponsive to a sense node 306 signal, which is in turn responsive tothe voltage across the current source terminals, formed by the emitterand collector terminals of transistor Q13, while the drain-sourcevoltage across power switch 304 MOSFET Q9 is changing, effectivelycontrolling the turn-on slew rate of power switch 304 MOSFET Q9. Inturn, the control of the power switch 304 MOSFET Q9 drain-source voltageslew rate controls the in-rush current flowing into bulk capacitor Cb.

The slew rate of power switch 304 MOSFET Q9 can be controlled bychanging the value of capacitor C1 or by changing the value of R27. Anadvantage of this example is that it does not require direct sensing ofthe current through power switch 304 MOSFET Q9 and therefore avoids theassociated losses and cost, that would be incurred by using a currentsense resistor or other direct means of sensing in-rush current. Thefact that a current source, rather than a simple resistor, is coupledbetween capacitor C1 and the input supply voltage terminal of thein-rush current limiting circuit 301 provides a stable current ensuringthat the signal generated at the sense node 306 is insensitive totemperature and input supply voltage variations in accordance with theteachings of the present invention.

In the foregoing detailed description, the method and apparatus of thepresent invention have been described with reference to a specificexemplary embodiment thereof. It will, however, be evident that variousmodifications and changes may be made thereto without departing from thebroader spirit and scope of the present invention. The presentspecification and figures are accordingly to be regarded as illustrativerather than restrictive.

1. A method of in-rush current limiting, comprising: receiving an inputsupply voltage signal from a power source coupled between inputterminals of an in-rush current limiting circuit; initiating a turn onof a power switch included in the in-rush current limiting circuit inresponse to the input supply voltage signal reaching a thresholdvoltage; initiating a turn on of a current source included in thein-rush current limiting circuit in response to the input supply voltagesignal reaching the threshold voltage; creating a current flow through acapacitor included in the in-rush current limiting circuit in responseto a voltage across the current source; and controlling a gate drive tothe power switch in response to the current flow through the capacitorto limit a voltage slew rate across the power switch.
 2. The method ofclaim 1, wherein creating the current flow through the capacitor furthercomprises conducting a current through a diode included in the in-rushcurrent limiting circuit in response to the current source turning on.3. The method of claim 2, wherein the diode has an anode and a cathodeand wherein the anode of the diode coupled to a drain terminal of thepower switch.
 4. The method of claim 2, wherein creating the currentflow through the capacitor further comprises maintaining a voltage at acathode of the diode such that the voltage at the cathode is one diodedrop below a voltage at a drain terminal of the power switch.
 5. Themethod of claim 1, wherein creating the current flow through thecapacitor further comprises creating the current flow through thecapacitor such that the current flow is responsive to a rate of changeof voltage between a drain and a source terminal of the power switch. 6.The method of claim 1, wherein creating the current flow through thecapacitor further comprises creating the current flow through thecapacitor such that the current flow is proportional to a rate of changeof voltage between a drain and a source terminal of the power switch. 7.The method of claim 1, further comprising determining the thresholdvoltage with a zener diode coupled between one of the input terminalsand a gate terminal of the power switch.
 8. The method of claim 1,wherein controlling the gate drive to the power switch comprises pullingdown a voltage on a gate terminal of the power switch in response to thecurrent flow through the capacitor.
 9. The method of claim 8, whereinpulling down the voltage on the gate terminal of the power switch tendsto turn off the power switch.
 10. The method of claim 1, wherein thein-rush current limiting circuit further includes a transistor, andwherein controlling the gate drive to the power switch comprises:generating a voltage signal with the current flow through the capacitor;and causing the transistor to conduct in response to the voltage signal.11. The method of claim 10, wherein the in-rush current limiting circuitfurther includes a resistor, wherein generating the voltage signalcomprises generating the voltage signal across the resistor with thecurrent flow through the capacitor.
 12. The method of claim 1, whereinthe power switch has a first terminal, a second terminal and a thirdterminal, wherein a terminal of the capacitor is coupled to one of theinput terminals of the in-rush current limiting circuit, and wherein aterminal of the current source circuit is coupled to the second terminalof the power switch and to another of the input terminals of the in-rushcurrent limiting circuit.
 13. The method of claim 1, wherein the powerswitch is a metal oxide field effect transistor (MOSFET).
 14. The methodof claim 1, wherein the power switch is a bipolar transistor.
 15. Themethod of claim 1, wherein the in-rush current limiting circuit iscoupled to a bulk capacitor such that an in-rush current flowing in thebulk capacitor is limited.
 16. The method of claim 15, wherein a firstterminal of the bulk capacitor is coupled to one of the input terminalsof the in-rush current limiting circuit and a second terminal of thebulk capacitor is coupled to a drain terminal of the power switch.